A Fish Movement Model for Assessing the Impacts of Total Dissolved Gas Supersaturation Downstream of High Dams

Total dissolved gas (TDG) supersaturation induced by high-dam discharge poses a severe threat to fish survival and represents a significant ecological risk in high-dam operations. Conducting fish survival risk assessments under TDG supersaturation is a critical component of environmentally friendly hydropower project construction and safe operation, serving as the scientific foundation for developing effective ecological mitigation measures. Based on historical behavioral experimental data of the endemic Upper Yangtze fish species, Schizothorax prenanti, under TDG supersaturation, combining random forest and hierarchical partitioning. this study identifies exposure time, TDG supersaturation level, and water temperature as the primary drivers influencing fish tolerance in TDG supersaturated water, and TDG supersaturation level and body length emerge as key determinants for avoidance capacity. Prediction formulas for mortality and horizontal avoidance rate were established based on these drivers. A fish tolerance model for TDG supersaturated water was constructed through instantaneous probability transformation, peak mortality definition, and non-negativity constraints. Fish movement behaviors were simplified into three core swimming vectors (random swimming, upstream migration, and horizontal avoidance). After calibrating the weight of each vector, a movement model for fish in TDG supersaturated water was constructed. This model was applied to a spawning ground downstream of a cascade hydropower station in the Yalong River to simulate fish movement trajectories and lethal effects in a dynamically changing TDG-supersaturated environment, including the TDG supersaturation levels experienced by fish, locations of death, and time of death. The results indicated fish possessed the ability to detect tributaries and utilize them to evade high TDG. Therefore, when formulating measures to mitigate the impacts of TDG supersaturation on fish, it is essential to fully consider the spatial patterns of river hydrodynamics and TDG distribution, suggesting to utilize tributaries to creat suitable habitats can enhance fish survival rates. This study marks the first application of fish behavioral patterns derived from laboratory experiments to river simulations. The findings provide technical support for developing fish protection measures downstream of high dams and assessing the ecological risks associated with TDG.